While a substantial proportion of women in the U.S. are considered at 'high risk' for breast cancer during their lifetime (Graubard 2010), most will never develop breast cancer. Women considered at high risk face mostly difficult choices to keep cancer at bay. Some are eligible for endocrine therapies such as Tamoxifen or Raloxifene, which have been associated with serious side effects in some users. Prevention options for high risk women are limited in part because our understanding of biological factors that cause breast cancer is fairly elementary. In addition, our current risk models only imprecisely estimate risk of subsequent cancer, and because effective and acceptable prevention options are scarce, most women at higher risk are not identified as such, or offered risk reducing therapies. Thus, there is a critical need to identify biomarkers in breast tissue that are related to subsequent risk, which may facilitate improved identification of women at high risk, and prompt development of therapies that are specific to their breast tissue changes. We propose to evaluate the contribution of protein biomarkers associated with early precancerous changes and tumor-initiating potential to subsequent risk of breast cancer among a large population-based cohort. Almost 15,000 women who received benign breast biopsies since 1996 were followed for breast cancer until 2009 by the New Mexico Tumor Registry, an NCI-sponsored SEER (Surveillance, Epidemiology, End Results) site that has been gathering information on cancer in New Mexico since 1968. A nested case-control study was conducted, and women who developed breast cancer were matched to three controls. An initial 85 breast cancer cases and 245 controls were evaluated for expression of 20 protein biomarkers in their benign biopsy tissue using immunohistochemistry. Included biomarkers are known to be active in pathways contributing to the earliest events in carcinogenesis. Our goal in this proposal is to complete the pathologist scoring of the immunohistochemical staining, analyze the data, and submit them for publication. Our efforts will include optimization of novel methods to determine the simultaneous presence of several biomarkers, to facilitate such assessment in future epidemiologic studies. Our study can serve two important purposes: (1) To improve risk models to more precisely distinguish women who will (and will not) go on to breast cancer; and (2) To identify biomarkers of malignant potential that may serve as targets for development of new prevention initiatives. Our goal in this grant is to more precisely distinguish women at high risk of breast cancer at least several years before pathologic detection of the tumor, when prevention might reasonably deter or delay its' development, and to provide information on biomarker expression in specific cancer-related pathways, which might guide tailored prevention efforts.